Gamma motor neurons (g-MNs) are a functionally and anatomically distinct subclass of neurons found in large number in most motor pools. These "fusimotor" neurons exclusively innervate the intrafusal fibers of the muscle spindle and comprise a parallel motor system that activates muscle spindle fibers independently of extrafusal muscle. The mechanisms that control the specification and differentiation of g-MNs are unknown, and no specific markers have been identified to distinguish g from a motor neurons in early development. We have found that g fusimotor neurons are selectively dependent on target muscle spindle-derived GDNF, and a recent study reports that fusimotor neurons are also selectively dependent on GDNF signaling in embryogenesis. Using this trophic requirement as a functional marker, we propose to perform a differential screen for genes selectively expressed in g-MNs using a mutant mouse in which g- MN precursors are selectively lost in the absence of GDNF signaling. We will also exploit this specific property of g-MNs in cultures of mouse embryonic stem cell-derived motor neurons to profile the pattern of gene expression in g-MNs in vitro. Finally, we will use molecular and size criteria established in our previous work to selectively isolate postnatal g-MNs for molecular analysis. The identification of genes specifically expressed in g-MNs at different stages of development will make possible a molecular genetic approach to study the formation and function of the fusimotor system, including its role in normal motor behaviors and disorders of motor control. PUBLIC HEALTH RELEVANCE: Gamma motor neurons regulate sensory feedback to the central nervous system from the periphery by controlling the sensitivity of stretch receptors in skeletal muscle. Their function is critical for motor control and for our perception of where our limbs are in space. This study will provide a molecular profile of gamma motor neurons, opening the way to future study of this motor system in health and disease.